1. Technical Field
The present invention relates to a thermal transfer sheet, in which a fluorescent agent has been incorporated for preventing the forgery/alteration of important papers, such as securities and paper money, and cards, such as ID cards and credit cards, and other articles having great asset values, or for improving the level of design and amusement, a thermal transfer method using said sheet, and a print produced using the same. More particularly, the present invention relates to a thermal transfer sheet that can produce a print which emits fluorescence with higher intensity and emits a more complicated fluorescent color and thus is difficult to forge and alter.
2. Prior Art
Various methods for preventing the forgery of securities, paper money, ID cards, credit cards and the like are known. Examples thereof include a method wherein fine characters or color figure patterns, which make copying difficult, are printed, a method wherein characters or images are formed using a transfer foil of gold or silver, which cannot be reproduced by three primary colors, or special colorants such as inks having a pastel tone or a pearl tone and fluorescent color inks, and a method wherein a hologram image, which can be formed only by an advanced production technique, is provided.
Further, a method has also been adopted wherein an image, which cannot be visually perceived under usual service environment, is formed using a fluorescent agent which does not substantially absorb visible light and is substantially colorless or white under visible light, but on the other hand, emits visible fluorescence upon the application of ultraviolet light, and the print is inspected with an ultraviolet lamp or the like for the presence of the fluorescent image to judge whether or not the print is genuine.
Japanese Patent Laid-Open No. 111800/1987 discloses a thermal transfer sheet using the above fluorescent agent. Further, Japanese Patent Laid-Open No. 207452/1996 discloses a thermal transfer sheet wherein thermally transferable dye layers of three primary colors of red, blue, green or four colors of the three primary colors and black and, in addition, a fluorescent color transfer layer containing a thermally transferable fluorescent dye have been provided in a mutually partitioned form on a continuous sheet.
In the prior art techniques, however, even when a fluorescent agent, which does not substantially absorb visible light and is substantially colorless or white under visible light, but on the other hand, emits visible fluorescence upon ultraviolet irradiation, is used, the forgery of the print is primarily possible by using quite or substantially the same colorant. In fact, color tones of currently known colorless fluorescent agents are roughly classified into three colors of red, blue, and green. For each color, color tones of fluorescent agents are similar to each other or one another even when they have been produced by different manufacturers. For example, for colorless fluorescent agents which emit red light, the emission wavelength is generally around 615 nm. Therefore, even for an identical color, it is difficult to visually distinguish one fluorescent agent from another fluorescent agent. For this reason, when a similar colorless fluorescent agent is available, the print can be in some cases forged without the use of the colorless fluorescent agent per se used in the “genuine print.”
Japanese Patent Laid-Open No. 125403/1995 discloses a method for forming a printed image which emits three or more fluorescent colors upon exposure to ultraviolet light, wherein images of two or more inks are printed, by thermal ink transfer using inks containing a fluorescent pigment or a fluorescent dye as a colorant which emits light upon exposure to ultraviolet light, on an object so as to partially overlap with each other.
Further, Japanese Patent Laid-Open No. 158823/2000 discloses a method for printing a fluorescent full-color image using a thermo-fusible (hot-melt) transfer sheet comprising inorganic colorless fluorescent agent transfer layers of a plurality of colors.
In these methods, however, since inks of a plurality of colors are printed so as to be superimposed on top of each other for the formation of a fluorescent full-color image, a multilayered structure of ink layers is formed on a part of the printing face. This poses a problem of deteriorated scratch resistance of the printed image.
Further, in the portion where the ink layers of a plurality of colors have been superimposed on top of each other, the quantity of ultraviolet light, which reaches the lower ink layer, is smaller than the quantity of ultraviolet light which reaches the upper ink layer. This results in lowered emission ability on the lower ink layer side and thus disadvantageously makes it difficult to regulate the color tone as desired by mixing of fluorescent colors.